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Are all kilojoules created equal?

Experts explain what kilojoules are, where you should get yours from and how many you need to maintain or lose weight.

Helen FosterSeptember 2018

Kilojoules are the way we measure the energy component of what we eat and drink, and the energy we use, each day. The original measure of this energy was the calorie, but in Australia we use the metric system of kilojoules. There are 4.2 kilojoules in every calorie.

You burn kilojoules simply by keeping your body going – breathing, thinking, physical activity and by your heart pumping blood around your muscles every time you move. When it comes to weight loss or gain, kilojoules count because you’ll put on weight if more kilojoules go in than out; that is, you consume more than you burn off each day.

All are kilojoules equal?

No. Rather than focusing purely on the kilojoules, consider the nutritional content of the food or drinks you choose.

“Take an avocado or nuts, they’re energy-dense [high kilojoule] foods but they’re full of healthy fats that keep the eyes, skin and brain healthy. An avocado has similar kilojoules to a small chocolate bar – but is far more nutritious,” says accredited practising dietitian Lisa Donaldson, spokesperson for the Dietitians Association of Australia.

To put it simply: one regular-sized avocado is 1210kJ. It contains at least 7% of the fibre, 25% of the folate and 30% of the potassium you need in a day – plus many other nutrients. It also contains around 12g of monounsaturated fat, which is shown to lower levels of the cholesterol in the body.

A small, 50g bar of milk chocolate can contain 1120kJ (almost the same as an avocado), but also has 23% of the fat and 25% of the sugar that’s recommended in a day. Most of the fat is saturated, which raises cholesterol. Although chocolate can contain fibre, potassium and iron, it’s comparatively low in nutrients.

As a general rule you should be focusing your kilojoule intake on wholefoods such as fruits, vegetables, wholegrains, lean meats, oily fish, nuts and seeds if you want the whole package for health.

How do you have a healthy diet, which doesn’t overdo the kilojoule intake?

Fat, protein and carbohydrates all contains a set number of kilojoules per gram. Fat contains 37.7kJ per gram, protein and carbohydrates each contain 16.7kJ per gram. This is one reason why cutting back on unhealthy fat automatically cuts down on the kilojoules you consume.

As well as limiting unhealthy fats – by avoiding the likes of deep-fried foods, processed foods and margarine – you need to lose foods known as ‘empty-calorie foods’.

“These are foods that supply energy to the body but few nutrients – lollies, chips, cake, soft drinks and alcohol all fall into this group,” says Donaldson.

The labels on the back of all processed foods give you details of the kilojoule count. You can track kilojoule content of foods through apps but it’s a good idea to check their data against the Australian Dietary Guidelines. A Sydney University study found considerable inaccuracy among weight loss apps. Among the Aussie apps that made the researchers’ top 10 list was ControlMyWeight by CalorieKing.

What should you focus on to lose weight?

If you’re trying to lose weight, which is better: limiting the number of kilojoules you eat or exercising? Both are important. Cutting kilojoules is far easier than burning them off. It takes just seconds to cut 420 kilojoules (by, say, skipping a handful of hot chips with your dinner) but to burn off that food once you’ve eaten it could take around 10 minutes of jogging.

“But simply decreasing kilojoule intake to control weight means you miss out on the health benefits of exercise like improving cardiovascular fitness and building muscle,” explains exercise physiologist Dr Jarrod Meerkin.

If you want to lose weight, you need to burn more energy than you consume. To lose weight at the rate of around half a kilo a week, it’s recommended you burn around 2,100kJ per day more than you consume.

How many kilojoules do you need in a day?

“You may have heard that 8,700 kilojoules is the average intake for a healthy Australian adult, but this is only a guide,” explains Donaldson. “A number of factors including your age, ethnicity, gender, body composition and level of physical activity all influence exactly how much you need.”

The best way to tell how many kilojoules you need in a day is to have a DEXA scan (dual energy X-ray absorptiometry), which measures your exact body composition and calculates your energy needs. These cost around $80. You can get one with an exercise physiologist or at a radiology clinic. You don’t need a doctor’s referral.

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